Zero-crossing detecting device and image forming device

ABSTRACT

A zero-crossing detecting device that detects a zero-crossing point of AC voltage, the device has a full-wave rectifier that rectifies the AC voltage and outputs a full-wave rectified voltage, a charger that is charged at a predetermined charging voltage by application of the full-wave rectified voltage, wherein the charger outputs a charging current when the full-wave rectified voltage falls below the charging voltage, and a signal output part that outputs a zero-crossing detecting signal. The signal output part outputs the zero-crossing detecting signal when the charging current flows to the signal output part.

CROSS REFERENCE TO RELATED APPLICATION

The invention is related to, claims priority from, and incorporates byreference Japanese Patent Application No. 2008-214220, filed on Aug. 22,2008.

TECHNICAL FIELD

The invention relates to a zero-crossing detecting device that detects apoint of zero charge (zero-crossing point) of alternating-current (AC)voltage, and an image forming device that has the zero-crossingdetecting device.

TECHNICAL BACKGROUND

An image forming device such as an electrographic printer is driven withAC power supply (AC voltage), and a power supply control for a heater toheat a fusing roller is made by using a switching element such as atriode ac (TRIAC).

A change in resistance of the heater for heating the fusing roller as afunction of temperature is wide, and an inrush current corresponding toa current that is several times that of a rated current may instantlyflow into the heater when the heater is cold and the internal resistanceis small. When the inrush current that is overcurrent flows into theheater, the heater may be overloaded and damaged.

Therefore, it is proposed that the image forming device have azero-crossing detecting circuit that detects the zero-crossing point ofAC voltage and outputs a zero-crossing signal each time thezero-crossing point is detected in order to control the switchingelement to turn ON when the AC voltage passes the point of zero charge(zero-crossing point) to reduce the inrush current to the heater. As thezero-crossing detecting circuit, for example as disclosed in Japaneselaid-open application 2006-258698, it is proposed that the zero-crossingdetecting circuit includes a first photocoupler, which detects apositive half-wave of AC voltage and outputs a square-wave correspondingto the half-wave, and a second photocoupler, which detects a negativehalf-wave of AC voltage and outputs a square-wave corresponding to thehalf- The proposed circuit outputs the zero-crossing signal by detectingthe zero-crossing point based on the square-waves output from eachphotocoupler.

FIG. 5 illustrates a related zero-crossing detecting circuit 300, andFIGS. 6A-6F illustrate signal waveforms that respective parts of therelated zero-crossing detecting circuit 300 output. In the relatedzero-crossing detecting circuit 300, when a current I1, which isrepresented by the waveform shown in FIG. 6B, flows into a photodiode(light-emitting element) in a photocoupler 301 based on the positivehalf-wave of an AC voltage E shown in FIG. 6A, the photodiode emitslight. The emitted light is received by a phototransistor(light-receiving element) in the photocoupler 301.

Since the phototransistor in the photocoupler 301 turns ON when itreceives the light from the photodiode mentioned above, it outputs anoutput signal F1, which is a square-wave shown in FIG. 6D, to a signalconverter 303.

Similarly, in the related zero-crossing detecting circuit 300, when acurrent I2, which is represented by the waveform shown in FIG. 6C, flowsinto a photodiode in a photocoupler 302 based on a negative half-wave ofthe AC voltage E shown in FIG. 6A, the photodiode emits light. Theemitted light is received by a phototransistor in the photocoupler 302.

The phototransistor in the photocoupler 302 turns ON by receiving thelight mentioned above, therefore an output signal F2, which is asquare-wave as shown in FIG. 6E, is output to the signal converter 303.

When the output signal F1, which is represented by the square-wave shownin FIG. 6D, and the output signal F2, which is represented by thesquare-wave shown in FIG. 6E, are input, the signal converter 303converts the output signals Fl and F2 respectively based on OR (logicalsum), and outputs a zero-crossing signal P0 that is a pulse signalshowing a rising timing of each signal. The waveform diagram of thezero-crossing signal P0 is illustrated in FIG. 6F.

However, since the related zero-crossing detecting circuit 300 mentionedabove is configured to send current constantly to the photocouplers 301and 302 so as to output the zero-crossing signal P0, there is a problemthat the zero-crossing detecting device or the image forming device inwhich it is installed consumes a relatively large amount of power.

In view of this problem, the invention provides a zero-crossingdetecting device and an image forming device including the zero-crossingdetecting device that are driven with low power consumption and are ableto detect a zero-crossing point of AC voltage.

SUMMARY

For solving the problem above, the invention employs the followingconfigurations. A first configuration is a zero-crossing detectingdevice that detects a zero-crossing point of AC voltage, a zero-crossingdetecting device in the present invention is characterized to include afull-wave rectifier that outputs a full-wave rectified voltage byrectifying AC voltage, a charger that is charged at a predeterminedcharging voltage by applying the full-wave rectified voltage and outputsa charging current when the full-wave rectified voltage becomes belowthe charging voltage, and a signal output part that outputs azero-crossing detecting signal by input of the charging current.

A second configuration is an image forming device that includes thezero-crossing detecting device of the first configuration.

With the zero-crossing point detecting device of these configurations,when a full-wave rectifier outputs a full-wave rectified voltage afterrectifying AC voltage, a charger is charged at a predetermined chargingvoltage by applying the full-wave rectified voltage until the AC voltageis in the neighborhood of a zero-crossing point. The zero-crossing pointdetecting device outputs a charging current when the AC voltage is in tothe neighborhood of the zero-crossing point and is below the chargingvoltage; therefore, the charger can output a zero-crossing signal bysending the charging current to a signal output part only for a shorttime in the neighborhood of the time of the zero-crossing point.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a circuit configuration of a zero-crossing detectingdevice of a first embodiment related to the invention.

FIG. 2 illustrates a configuration of the zero-crossing detecting deviceof the embodiment of FIG. 1.

FIGS. 3A-3D illustrate signal waveforms that are output from each partof the zero-crossing detecting device of the embodiment of FIG. 1.

FIG. 4 illustrates a heater control circuit of a fuser in an imageforming device, which is an example of an application of the circuit ofFIG. 1.

FIG. 5 illustrates a related zero-crossing detecting circuit.

FIGS. 6A-6F illustrate signal waveforms that are output from parts ofthe related zero-crossing detecting circuit of FIG. 5.

FIGS. 7A-7D illustrate signal waveforms that are output when asuperimposed noise is mixed into a commercial power supply of thezero-crossing detecting device of the embodiment of FIG. 1.

FIG. 8 illustrates a configuration of a zero-crossing detecting deviceof a second embodiment of the invention.

FIG. 9 illustrates a circuit configuration of the zero-crossingdetecting device of the second embodiment of FIG. 8.

FIGS. 10A-10D illustrate signal waveforms that are output from parts ofthe zero-crossing detecting device of FIG. 8.

DETAILED DESCRIPTION

Preferred embodiments of the invention are described in detail withreference to the drawings.

Embodiment 1

FIG. 2 is a block diagram showing a configuration of a zero-crossingdetecting device 100 of the first embodiment. The zero-crossingdetecting device 100 consists of a commercial power supply 1, afull-wave rectifier 2, a discharger 3, a charger 4, a gate part 5 (orgate means), and a zero-crossing signal output sensor 6 (orzero-crossing signal output part), as shown in FIG. 2.

FIG. 1 is a circuit diagram showing the configuration of thezero-crossing detecting device 100, and FIGS. 3A-3D illustrate signalwaveforms that are output from parts of the zero-crossing detectingdevice 100. The commercial power supply 1 is an AC power supply providedfrom an electric power company for industrial use and for home use. TheAC power supply provides an AC voltage E, which is represented by thewaveform shown in FIG. 3A, to the full-wave rectifier 2.

The full-wave rectifier 2 has a bridge circuit that consists of diodesD1, D2, D3, and D4, rectifies the full-wave of the AC voltage E usingthe bridge circuit, and outputs a full-wave rectified voltage A, whichis represented by the solid waveform shown in FIG. 3B.

The discharger 3 has a resistance R1 as shown in FIG. 1.

On the other hand, when a current I7 mentioned below is provided from agate part 5 as shown in FIG. 1, the discharger 3 discharges the currentI7 to a ground by using the resistance R1.

The charger 4 consists of a diode D5, a zener diode ZD1, a resistance R2for preventing an inrush current, and a capacitor C1 for storingelectric charge to provide it to the zero-crossing output sensor 6, asshown in FIG. 1. In the charger 4, a current based on the full-waverectified voltage A mentioned above flows to the diode D5 when thefull-wave rectified voltage A is over a voltage V1 shown in FIG. 3B, Thevoltage V1 is determined by a constant number of the zener diode ZD1,the resistance R2, and the capacitor C1. The current that passes throughthe diode D5 flows through the zener diode ZD1, the resistance R2, andthe capacitor C1 in that order, and is charged as electric charge in thecapacitor C1.

When the capacitor C1 starts storing the electric charge, a chargingvoltage B is represented by the dotted line waveform shown in FIG. 3B.The capacitor C1 continues storing charge until the charging voltage Bbecomes a saturation voltage V2 shown in FIG. 3B.

After that, as shown in FIG. 3B, the capacitor C1 discharges the storedelectric charge when the full-wave rectified voltage A falls accordingto a power supply period of the AC voltage E, and falls below thecharging voltage B of the capacitor C1. That is, the capacitor C1discharges when the voltage E is in the neighborhood of thezero-crossing point. The discharged electric charge is provided to thezero-crossing signal output sensor 6 by the gate part 5.

The gate part 5, which consists of a diode D6, is used to transmit thedischarged electric charge by the capacitor C1 of the charger 4 to aphotodiode of a photocoupler 7 in the zero-crossing signal output sensor6. The photocoupler 7 includes a photodiode, a phototransistor, and anuninterrupted light path between the photodiode and the phototransistor,as shown in FIG. 1. The current I7 that flows into the gate part 5 isoutput to the discharger 3 after passing through the photodiode of thephotocoupler 7.

The zero-crossing signal output sensor 6 consists of the photocoupler 7and a pull-up resistance R3 for preventing voltage fluctuation of adirect-current (DC) power supply 5V. That is, in the zero-crossingsignal output sensor 6, the electric charge is provided from the charger4 by the gate part 5, and the photodiode emits light when the currentI7, which has the waveform shown in FIG. 3C, flows to the photodiode ofthe photocoupler 7. The emitted light is received by the phototransistorof the photocoupler 7.

Since the phototransistor in the photocoupler 7 turns ON only for ashort time by receiving the light mentioned above, it outputs a currentfrom the 5V DC power supply to form a square-wave, or pulse,zero-crossing detecting signal P, which is synchronized with the timingof the AC voltage E passing through the zero-crossing point as shown inFIG. 3D.

According to the zero-crossing detecting device 100 of the embodiment ofFIG. 1, since the current I7 does not flow to the photocoupler 7 untilthe full-wave rectified voltage A is in the neighborhood of thezero-crossing point, and the phototransistor does not emit light most ofthe time, the zero-crossing detecting signal P is provided by using arelatively small amount of power.

The zero-crossing detecting device 100 of FIG. 1 is installed in adevice such as an image forming device that controls a provided powerbased on the zero-crossing detecting signal P by a phase control. Forexample, an image forming device that has the zero-crossing detectingdevice 100 performs heating control of a heater placed inside a heatingroller of a fuser based on the zero-crossing detecting signal P outputfrom the device, as described below. The image forming device can be acolor printer, a black and white printer, a copier, a fax machine, andMulti Function Peripheral (MFP), for example.

FIG. 4 illustrates a configuration of a heater control circuit 200 of afuser in an image forming device having the zero-crossing detectingdevice 100 of the embodiment 1 related to the invention. The imageforming device controls the timing to operate the heater mentioned aboveby using the heater control circuit 200 shown in FIG. 4.

The heater control circuit 200 consists of a heater 201 in the heatingroller of the fuser (not shown), a photo-TRIAC 202, a TRIAC 203, and anAC power supply 204, as shown in FIG. 4.

The heater control circuit 200 is controlled by a CPU of the imageforming device. The CPU outputs a pulse signal S to the photo-TRIAC 202of the heater control circuit 200 in order to heat the heater insynchronism with the rising of the zero-crossing detecting signal Pshown in FIG. 3D, which is output from the zero-crossing detectingdevice 100. Here, the CPU controls an output time of the pulse signal Setc. based on a surface temperature that is detected by a temperaturesensor to detect the surface temperature of the heating roller of thefuser.

The photodiode of the photo-TRIAC 202 emits light when the pulse signalS is input. The emitted light is received by a TRIAC in the photo-TRIAC202.

The TRIAC in the photo-TRIAC 202 turns ON when it receives the lightfrom the photodiode, which causes a TRIAC 203 to conduct.

When the TRIAC 203 conducts, a current based on an AC voltage that isoutput from an AC power supply 204 flows into a heater 201, and theheater generates heat and heats the heating roller of the fuser frominside.

An image forming device having the zero-crossing detecting device andthe heater control circuit 200 of FIG. 4, the zero-crossing detectingdevice 100 can be driven with a low power consumption. Therefore, thedevice can be driven with relatively low power consumption.

Embodiment 2

Compared with the configuration of the zero-crossing detecting device100 of FIG. 1, a zero-crossing detecting device 100 a of FIGS. 8 and 9is configured to add a zener diode ZD2 to a gate part 5 a for preventingthe detection of noise and accidental output of noise detection as thezero-crossing detecting signal when the noise is mixed in an AC voltageE output from a commercial power supply 1.

FIGS. 7A-7D illustrate signal waveforms output from each part of thezero-crossing detecting device 100 in the first embodiment of FIG. 1when noise is mixed in the output of the commercial power supply 1. Asshown in FIG. 7A, when noise G occurs in the AC voltage E output fromthe commercial power supply 1, the full-wave rectifier 2 outputs afull-wave rectified voltage A with the noise shown in FIG. 7B based onthe AC voltage E.

When a current based on the full-wave rectified voltage A via thedischarger 3 is input, the charger 4 charges the capacitor C1 via thediode D5, the zener diode ZD1, and the resistance R2.

When the full-wave rectified voltage A becomes smaller than the chargingvoltage B because of noise at the time of charging the capacitor C1 asshown in FIG. 7B, a photodiode emits light since a current (electriccharge) is provided from the charger 4 and the charging current flowsinto the photodiode of the photocoupler 7.

Since the phototransistor in the photocoupler 7 turns ON by receivingthe light signal from the associated photodiode, it outputs a noisesignal caused by a noise part of the AC voltage E shown in FIG. 7D. Thezero-crossing detecting device 100 of FIG. 1 may detect the noise partas a zero-crossing point when the noise is mixed into the AC voltage Eas shown.

FIG. 8 is a block diagram that illustrates a configuration of thezero-crossing detecting device 100 a of the second embodiment. Thezero-crossing detecting device 100 a in FIG. 8 consists of thecommercial power supply 1, the full-wave rectifier 2, the discharger 3,the charger 4, a gate part 5 a, and the zero-crossing signal outputsensor 6 as shown, for preventing detection of the noise part of the ACvoltage E as a zero-crossing point.

As shown in FIG. 9, the gate part 5 a consists of a diode D6 and a zenerdiode ZD2 that has a zener voltage ΔV. As shown in FIG. 10B, when thefull-wave rectified voltage A becomes lower than the charging voltage Bof the capacitor C1 and the zener voltage ΔV of the zener diode ZD2, thecapacitor C1 applies the charging voltage B (which is larger than thezener voltage ΔV) to the zener diode ZD2 in order to start dischargingby the charging voltage B. By this, the zener diode ZD2 conducts and thecurrent I7 is output to the photocoupler 7 as shown in FIG. 10C.

On the other hand, when a noise component is superimposed on thefull-wave rectified voltage A, the zener diode ZD2 does not conduct andthe electric charge is not provided to the photodiode of thephotocoupler 7 from the capacitor C1, since the full-wave rectifiedvoltage A is only added as a voltage below the zener voltage ΔV to thezener diode ZD2 because of the noise as shown at G in FIGS. 10B and 10C.Therefore, the noise part is not detected as a zero-crossing point. Thezener voltage ΔV of the zener diode ZD2 is a slightly smaller value thanthe charging voltage B.

FIG. 9 illustrates a circuit configuration of the zero-crossingdetecting device 100 a of the second embodiment, and FIGS. 10A-10Dillustrate signal waveforms output from each part of the zero-crossingdetecting device 100 a in the second embodiment. Like the zero-crossingdetecting device 100 of the first embodiment, the AC voltage E shown inFIG. 10A is output from the commercial power supply 1, and the full-waverectified voltage A shown in FIG. 10B is output by rectifying thefull-wave of the AC voltage E in the full-wave rectifier 2. Then, acurrent based on the full-wave rectified voltage A is provided to thecharger 4 via the discharger 3, and the capacitor C1 continues to storecharge until the charging voltage B reaches a saturation voltage V2 asshown in FIG. 10B.

After that, as shown in FIG. 10B, when the full-wave rectified voltage Afalls according to the power supply period of the AC voltage E, close tothe zero-crossing point, and falls below the charging voltage B of thecapacitor C1 and the zener voltage ΔV of the zener diode ZD2, that is,when the voltage A is in the neighborhood of the zero-crossing point,the capacitor C1 applies the charging voltage B to the zener diode ZD2in order to start discharging by the charging voltage B. As a result, asshown in FIG. 10C, the zener diode ZD2 is caused to conduct, and thecurrent I7 is output to the photocoupler 7.

In the zero-crossing signal output sensor 6, when the electric charge isprovided from the charger 4 via the gate part 5 a and the square-wavecurrent I7 shown in FIG. 10C flows into the photodiode of thephotocoupler 7, the photodiode emits light. The emitted light isreceived by the phototransistor of the photocoupler 7. The current I7that flows into the gate part 5 a after passing through the photodiodeof the photocoupler 7 is output to the discharger 3.

Since the phototransistor of the photocoupler 7 turns ON only for ashort time by receiving the light mentioned above, the zero-crossingsignal output sensor 6 outputs a current from the DC 5V power supply toform a square-wave, zero-crossing detecting signal P, which issynchronized with the timing of the AC voltage E passing through thezero-crossing point as shown in FIG. 10D.

On the other hand, as shown in FIGS. 10A and 10B, when noise G issuperimposed on the voltage E, since the full-wave rectified voltage Ais smaller than the zener voltage ΔV, the zener diode ZD2 does notconduct; therefore, the current I7 does not flow and the noise signaldoes not affect the output of the signal P.

Otherwise, the configuration of the zero-crossing detecting device 100 aof the second embodiment 2 is the same as that of the zero-crossingdetecting device 100 of the first embodiment.

Also, the configuration of the heater heating control placed inside theheating roller of the fuser in the image forming device having thezero-crossing detecting device 100 a of the second embodiment is thesame as that of the heater heating control in the image forming devicehaving the zero-crossing detecting device 100 of the first embodiment inthe exemplary application described above.

According to the zero-crossing detecting device 100 a of the secondembodiment, when noise is mixed in the commercial power supply 1 and anoise component is superimposed onto the full-wave rectified voltage A,the zener diode ZD2 does not conduct since the full-wave rectifiedvoltage A is only added to the zener diode ZD2 as a voltage below thezener voltage ΔV, the current I7 does not flow in the photodiode sincethe electric charge is not provided from the capacitor C1 to thephotodiode of the photocoupler 7, and the zero-crossing detecting signalP based on the zero-crossing point can be output without detecting thenoise component since the phototransistor does not emit light.

Although it is described to use the zener diode ZD2 as the gate part 5 ain the second embodiment, there is no need to limit the gate part to azener diode, and a DIAC, for example, may be employed instead.

1. A zero-crossing detecting device that detects a zero-crossing pointof AC voltage, the device comprising: a full-wave rectifier thatrectifies the AC voltage and outputs a full-wave rectified voltage; acharger that is charged at a predetermined charging voltage byapplication of the full-wave rectified voltage, wherein the chargeroutputs a charging current when the full-wave rectified voltage fallsbelow the charging voltage; and a signal output part that outputs azero-crossing detecting signal, wherein the signal output part outputsthe zero-crossing detecting signal when the charging current flows tothe signal output part.
 2. The zero-crossing detecting device accordingto claim 1, wherein the charger comprises at least a diode, a zenerdiode, and a capacitor.
 3. The zero-crossing detecting device accordingto claim 1, wherein the signal output part comprises at least onephotocoupler.
 4. The zero-crossing detecting device according to claim1, further comprising a gate part that flows the charging current beingoutput by the charger to the signal output part.
 5. The zero-crossingdetecting device according to claim 4, wherein the gate part comprisesat least one diode.
 6. The zero-crossing detecting device according toclaim 4, wherein the gate part comprises at least one diode and onezener diode.
 7. The zero-crossing detecting device according to claim 1,further comprising a discharger that discharges the charging currentinput from the signal output part to a ground of the full-waverectifier.
 8. The zero-crossing detecting device according to claim 7,wherein the discharger comprises at least one resistance.
 9. Thezero-crossing detecting device according to claim 1, wherein thezero-crossing detecting device is installed in an image forming device.10. The zero-crossing detecting device according to claim 1, wherein thesignal output part is a sensor that senses the output of the chargingcurrent.
 11. The zero-crossing detecting device according to claim 1,wherein the zero-crossing detecting signal is a pulse signal.
 12. Azero-crossing detecting device that detects a zero-crossing point of ACvoltage, the device comprising: a full-wave rectifier that rectifies theAC voltage and outputs a full-wave rectified voltage; a charger circuitto which the full-wave rectified voltage is applied, wherein the chargercircuit outputs a charging current when the AC voltage is approximatelyzero; and a signal output sensor that outputs a zero-crossing detectingsignal, wherein the signal output sensor outputs the zero-crossingdetecting signal only when the charging current flows to the signaloutput sensor.
 13. The zero-crossing detecting device of claim 12,wherein the charger circuit is charged at a predetermined chargingvoltage by the full wave rectified voltage, and the charger circuitoutputs the charging current when the full-wave rectified voltage fallsbelow the charging voltage.
 14. A method of detecting a zero-crossingpoint of AC voltage comprising: rectifying the AC voltage and outputtinga full-wave rectified voltage; applying the full-wave rectified voltageto a charger circuit; storing a charge at a predetermined chargingvoltage in the charger circuit; outputting a charging current from thecharger circuit when the full-wave rectified voltage falls below thecharging voltage; and outputting a zero-crossing detecting signal whenthe charging current flows to the signal output device to indicate timeswhen the AC voltage is approximately zero.
 15. The method of detecting azero-crossing point of AC voltage according to claim 14, wherein themethod comprises outputting a pulse signal as the zero-crossingdetecting signal.